In focus tilt control in a scan exposure apparatus, a drive profile for a wafer stage is calculated to align the position of a wafer surface with an image focus surface of a reticle plate. Regarding this technique, Japanese Patent Application Laid-Open No. 2002-222760 discloses a method of holding a stage drive profile for each reticle. The distortion of a reticle in the X and Y directions is previously measured and a stage drive profile for a currently-exposed stage is generated based on the measurement.
Further, Japanese Patent Application Laid-Open No. 6-36987 discloses a method of obtaining deflection and tilt of a reticle held on a reticle stage by irradiating a pattern on a reference member on the wafer stage side from a position thereunder and detecting light reflected from the reticle as a photoelectric signal, thereby obtaining an optimum image surface. Further, Japanese Patent Application Laid-Open No. 7-272999 discloses a method of measuring the position of a reticle surface while scanning the reticle with plural focus measurement sensors arranged in a non-scan direction, and measuring focus and shape in a tilt direction of the reticle attracted to a reticle stage.
In the above-described conventional techniques, the measurement of a reticle shape is made by emitting measurement light on the surface of the reticle. However, as a pattern exists on the reticle surface, the reticle shape is measured by actually emitting the measurement light on the reticle pattern surface. In this case, erroneous measurement of a focus value caused by misalignment of the reticle pattern drawn on the reticle causes an error, which may degrade the accuracy of the reticle shape measurement. The error of alignment of the reticle pattern surface in a focus direction, by one over the projection magnification to the power of two, influences an image focus surface in the focus direction. Generally, the thickness of a chrome pattern drawn on a reticle is about 100 to 200 nm, and through a one-quarter scale projection optical system, an error up to 6.25 to 12.5 nm may be generated. For example, the best focus width of an ArF scan exposure apparatus with a line width of 0.08 μm (L/S) is about 300 nm. The error allocated to a focus positioning factor upon exposure is 40 to 50 nm, which cannot be ignored. The error of the focus value degrades the resolution performance upon scan exposure.